Ceramic wash-coat for catalyst support

ABSTRACT

A wash-coat ( 16 ) for use as a support for an active catalyst species ( 18 ) and a catalytic combustor component ( 10 ) incorporating such wash-coat. The wash-coat is a solid solution of alumina or alumina-based material (Al 2 O 3 -0-3 wt % La 2 O 3 ) and a further oxide exhibiting a coefficient of thermal expansion that is lower than that exhibited by alumina. The further oxide may be silicon dioxide (2-30 wt % SiO 2 ), zirconia silicate (2-30 wt % ZrSiO 4 ), neodymium oxide (0-4 wt %), titania (Al 2 O 3 -3-40% TiO 2 ) or alumina-based magnesium aluminate spinel (Al 2 O 3 -25 wt % MgO) in various embodiments. The active catalyst species may be palladium and a second metal in a concentration of 10-50% of the concentration of the palladium.

GOVERNMENT INTEREST

This invention was made with United States Government support throughContract Number DOE-DE-FC26-03NT41891 awarded by the Department ofEnergy, and, in accordance with the terms set forth in that contract,the United States Government may have certain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to the field of ceramics and moreparticularly to ceramic catalyst supports for use in combustors of gasturbine engines.

BACKGROUND OF THE INVENTION

Catalytic combustion presents a viable alternative to conventional flamecombustion for addressing environmental issues for power generators suchas gas turbine engines. A catalytic combustion system typically includesan active coating, such as a noble metal in a thermally stable ceramicmatrix, deposited onto a metallic support. U.S. Pat. No. 4,870,824describes a catalytic combustor wherein a first surface of a metallicconduit is coated with an active catalytic material such as palladium ona stabilized alumina (γ—Al₂O₃) wash-coat for causing combustion in afuel/air plenum, while an opposed surface of the conduit has no suchcatalytic material and functions as a heat-removal surface to cool theconduit material.

The role of the alumina wash-coat is to provide a high surface area forexposure of the combustible mixture to the noble metal constituent. Thecatalyst initiates the combustion reaction at its surface at atemperature that is lower than a normal flame ignition temperature.However, once the reaction is initiated, it continues in the gas streamand persists beyond the catalyst in the form of afterburning.Simultaneously, the catalyst substrate temperature increases, resultingin an accelerated reaction that moves the reaction zone further upstreamin the gas stream. The reaction must be controlled to avoidover-temperature damage to the catalyst material or to the substratematerial.

The behavior of the catalyst system is primarily controlled by theproperties of the active noble metal at lower temperatures. For example,the light-off temperature will be a function of the concentration of thenoble metal. However, at very high temperatures, the behavior of thesystem will be controlled primarily by the properties of thewash-coat/support. For example, the surface area, reactivity, porosity,thermal conductivity and mechanical integrity of the wash-coat atelevated temperatures must be considered. The performance of thewash-coat may be a limiting factor in the design of a modem hightemperature catalytic combustor.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole figure is a partial cross-sectional view of a catalyticcombustor showing a metallic support structure coated with an improvedcombustion catalyst system.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have evaluated prior art combustion catalystsystems and have found that the known γ-Al₂O₃ wash-coat is adequate forcertain applications, for example the backside-cooled combustor of U.S.Pat. No. 4,870,824. However, such a known alumina wash-coat exhibitsseveral characteristics that may limit its effectiveness in very hightemperature applications, such as may be desired for the next generationof catalytic combustors. In particular, the current alumina wash-coatsystem suffers from poor phase stability, surface area loss at hightemperature, and rapid surface diffusion causing catalyst agglomerationand may be unacceptable for applications where the material temperaturemay exceed 800° C. The γ-Al₂O₃ phase with a specific surface area (SSA)of 125-250 m²/g transforms into either θ or δ phase with an SSA of 18-30m²/g at 450° C., which then transforms to α phase with an SSA of 5 m²/gbetween 900-1100° C. depending upon the originating phase (either θ or δphase). Surface diffusion occurs concurrently above 800° C.

The present inventors have discovered that improved catalyst systemperformance may be achieved by the addition of a second or more oxides,such as an oxide exhibiting a lower coefficient of thermal expansionthan that of alumina, to the stabilized alumina wash-coat. Literaturereports addition of La₂O₃ and BaO to stabilize the surface area ofalumina and the addition of CeO₂ and NiO to prevent the transformationof PdO to Pd. In various embodiments of the present invention, silicondioxide (SiO₂), zirconia silicate (ZrSiO₄), neodymium oxide (Nd₂O₃),and/or titanium dioxide (TiO₂) are added to the traditional aluminawash-coat as well as to the La₂O₃ modified Al₂O₃ wash-coat system. Thesemodified alumina-based wash-coats may be referred to as dopedalumina(s). As a result of a lowered thermal expansion, improved phasestability (both wash-coat and PdO) to higher temperatures and improvedchemical properties, these novel multi-oxide systems retain theirsurface area at elevated temperatures, provide excellent thermal shockresistance, and exhibit reduced agglomeration of the active species anda reduced tendency for support structure cracking, thus preservingadhesion to the substrate and improved catalyst deactivation resistance.The net effect is a more durable catalyst with higher conversionefficiency than the prior art alumina wash-coat system.

Alternatively, magnesium aluminate (MgAl₂O₄) spinel can be used aswash-coat. The spinel system is sintering resistant, highly active,exhibits high melting temperature, excellent chemical stability, lowacidity and good mechanical strength. The spinel phase is stable (nophase transformation) to the melting point. Furthermore, the spinelphase is an alumina-based system that is compatible with a thermallygrown oxide support substrate interface leading to reduced mismatchstresses. This new wash-coat material may be referred to as a stablewash-coat.

The sole figure illustrates a component 10 of a gas turbine enginecombustor incorporating the present combustion catalyst system 12,wherein an underlying metallic support structure 14 is coated with alayer of an improved ceramic catalyst support 16 within which isdistributed an active catalyst species 18. The catalyst species 18 maybe a noble metal or other catalytic material known in the art. Thecatalyst species 18 may be uniformly distributed throughout the catalystsupport 16 or there may be a concentration gradient across a depth ofthe catalyst support layer 16, such as from a higher concentration ofthe catalyst species 18 proximate an outermost surface 20 that isexposed to the fuel/air mixture to a lower concentration proximate theunderlying support structure 14. Such non-uniform loading of thecatalyst species 18 may be useful for lowering the cost of the catalystsystem 12 while still providing adequate combustion performance for aspecified design lifetime. The loading of the catalyst species 18whether uniform or graduated may be in the range of 2-25 mg/in², forexample.

Furthermore, the catalyst species 18 may include more than one catalyticmaterial, such as including both palladium 18 a and a second metal 18 b,such as at least one of the group of platinum, rhodium, ruthenium, gold,nickel and rhenium. Palladium is often selected as a catalyst for thecombustion of methane due to its low light-off temperature, resistanceto thermal and hydrothermal sintering and volatilization, and durabilitydue to its capability for self-control with reversible PdO—Pdtransformation. The addition of a second or more metals may addressissues related to sintering and poisoning of the palladium catalyst. Themechanisms for better stability of the bi-metallic catalyst may beattributed to influence on the Pd—PdO transformation and activity(increased stability of PdO phase) and catalyst particle size growth(for platinum and rhodium), retention of dispersion of the palladiumcrystallites (for ruthenium), or improved resistance to sulfur,phosphorus and nitrogen poisoning (for rhenium), for example. Theconcentration of the second active catalyst can range from 10-50% of thepalladium concentration, depending on the operational requirements.

In one embodiment, the ceramic catalyst support 16 may incorporatesilicon dioxide in addition to alumina or alumina-La₂O₃. The silicondioxide may be in the range of 2-30% by weight of the alumina and can berepresented as a) Al₂O₃-2-30 wt % SiO₂ or b) Al₂O₃-0-3 wt % La₂O₃-2-30wt % SiO₂ (i.e. ratio of Al₂O₃ weight of 100: to La₂O₃ weight of 0-3:toSiO₂ weight of 2-30). The addition of SiO₂ results in relatively smallerloss of catalyst mass together with a lack of cracking (SiO₂ being asintering aid for Al₂O₃) that results in better adhesion. Also theaddition results in stabilization of the surface area of alumina.

In another embodiment, the ceramic catalyst support 16 may incorporatezirconia silicate in addition to alumina or alumina-La₂O₃. The zirconiasilicate may be in the range of 2-30% by weight of the alumina and canbe represented as a) Al₂O₃-2-30 wt % ZrSiO₄ or b) Al₂O₃-0-3 wt %La₂O₃-2-30 wt % ZrSiO₄. The addition of ZrSiO₄ results in increasedcatalytic activity due to the stabilization of the reactive PdO phasefrom transforming to metallic Pd. It also substantially raises thethermal shock resistance and reduces active catalyst particleagglomeration.

In a further embodiment, the ceramic catalyst support 16 may incorporateNd₂O₃ in addition to alumina or alumina-La₂O₃. The neodymium oxide maybe in the range of 0-4% by weight of the alumina and can be representedas a) Al₂O₃-0-4 wt % Nd₂O₃ or b) Al₂O₃-0-3 wt % La₂O₃-0-4 wt % Nd₂O₃.The addition of Nd₂O₃ retards active catalyst particle growth andincreases lifetime. This is not due to stabilization of the surface areaof alumina, but rather due to the prevention of the decrease in numberof active sites as well as the prevention of the transformation of PdOto Pd in the reaction. During combustion of a fuel in the presence ofthe catalyst, this provides an increased NO conversion to N² andpromotes the formation of CO and H² and prevents the formation of HCN,NH₃ and carbon formation at high temperatures. In this case, there is asynergistic effect due to La₂O₃ and Nd₂O₃ that is more prominent thanfor the addition of Nd₂O₃ only.

In a further embodiment, the ceramic catalyst support 16 may incorporateTiO₂ in addition to alumina or alumina-La₂O₃, owing to its excellenthigh temperature properties. The titania may be in the range of 3-40% byweight of the alumina and can be represented as a) Al₂O₃-3-40 wt % TiO₂or b) Al₂O₃-0-3 wt % La₂O₃-3-40 wt % TiO₂. The titania doping aids instabilizing gamma alumina, which is the phase of interest from thecatalytic activity and surface reactivity aspect. The alumina-titaniasystem also exhibits superior properties in terms of operatingtemperatures, chemical stability and low thermal expansion when comparedto the prior art alumina wash-coat.

In a further embodiment, the ceramic catalyst support 16 may incorporatean alumina-based magnesium aluminate spinel. The magnesium aluminatespinel is a stoichiometric concentration (Al₂O₃-25 wt % MgO).

The improved ceramic catalyst support 16 may be deposited onto themetallic support structure 14 by known processes such as sol gel orthermal spray. The ceramic materials may be co-sprayed with one or moreceramicious binders and the active catalyst species 18 in a solution orpowder form. Alternatively, the active catalyst species 18 may beintroduced to a deposited ceramic catalyst support 16 layer by incipientwetting of the precious metal. When using doped or stable wash-coatcompositions, the defined compositions is initially formed and thedeposited on the substrate.

While various embodiments of the present invention have been shown anddescribed herein, it will be obvious that such embodiments are providedby way of example only. Numerous variations, changes and substitutionsmay be made without departing from the invention herein. Accordingly, itis intended that the invention be limited only by the spirit and scopeof the appended claims.

1. A wash-coat for use as a support for an active catalyst species, thewash-coat comprising one of alumina and an alumina-based material, plus0-3 wt % La₂O₃, and a further oxide other than La₂O₃, BaO, CeO₂ and NiO,the further oxide exhibiting a coefficient of thermal expansion that islower than that exhibited by alumina.
 2. The wash-coat of claim 1,wherein the further oxide comprises silicon dioxide.
 3. The wash-coat ofclaim 2 comprising 2-30 wt % SiO₂.
 4. The wash-coat of claim 1, whereinthe further oxide comprises zirconia silicate.
 5. The wash-coat of claim4 comprising 2-30 wt % ZrSiO₄.
 6. The wash-coat of claim 1, wherein thefurther oxide comprises neodymium oxide.
 7. The wash-coat of claim 6comprising 0-4 wt % Nd₂O₃.
 8. The wash-coat of claim 1, wherein thefurther oxide comprises titanium oxide.
 9. The wash-coat of claim 9comprising Al₂O₃-3-40 wt % TiO₂.
 10. The wash-coat of claim 1 comprisingalumina-based magnesium aluminate spinel in a stoichiometricconcentration of Al₂O₃-25 wt % MgO.
 11. A component for a catalyticcombustor comprising: a metallic substrate; a ceramic catalyst supportdisposed on the metallic substrate; and an active catalytic speciesdisbursed within the ceramic catalyst support; wherein the ceramiccatalyst support comprises one of alumina and an alumina-based material,plus 0-3 wt % La₂O₃, and a further oxide other than La₂O₃, BaO, CeO₂ andNiO, the further oxide exhibiting a coefficient of thermal expansionthat is lower than that exhibited by alumina.
 12. The component of claim11, wherein a concentration of the active catalytic species variesacross a depth of the ceramic catalyst support.
 13. The component ofclaim 11, wherein the active catalyst species further comprisespalladium at a first concentration and a second metal at a secondconcentration of 10-50% of the first concentration.
 14. The component ofclaim 11, wherein the active catalyst species further comprisespalladium and at least one of the group of platinum, rhodium, ruthenium,gold, nickel and rhenium.
 15. The component of claim 11, wherein thefurther oxide comprises silicon dioxide.
 16. The component of claim 11,wherein the further oxide comprises zirconia silicate.
 17. The componentof claim 11, wherein the further oxide comprises neodymium oxide. 18.The component of claim 11, wherein the further oxide comprises titaniumoxide.
 19. The component of claim 11, wherein the ceramic catalystsupport comprises alumina-based magnesium aluminate spinel in astoichiometric concentration of Al₂O₃-25 wt % MgO.
 20. The component ofclaim 19, wherein the active catalyst species further comprisespalladium at a first concentration and a second metal at a secondconcentration of 10-50% of the first concentration.